Early differential mechanisms consisted of a set of planetary gears coupled between two half-shafts of a drive axle. Such a drive axle has the advantages over a solid axle that the wheels of the vehicle can travel at different speeds and equal driving force can be applied to the driving wheels. However, under certain driving conditions, this conventional differential has a serious deficiency. For example, if a drive wheel is on a slippery surface, such as ice or mud, that wheel will slip and spin, and will not provide significant driving torque to move the vehicle. The other drive wheel, which well may be on a surface that the tire will not slip, can apply no more driving torque than the spinning wheel because the differential delivers only an equal amount of torque to both wheels. Thus, the total driving force can never be more than twice the amount applied by the wheel with the poorest road adhesion.
Traction is also adversely effected, especially during hard driving, by other conditions that unbalance the weight on the driving wheels. When driving at high speed around a curve, the weight is transferred from the inside wheel to the outside wheel. Hard acceleration coming out of a turn can then cause the inside wheel to spin because it has less weight on it and therefore less road adhesion. Similarly, during any quick acceleration there is propeller shaft reaction torque on the rear axle assembly. When one wheel is partially unloaded and loses part of its traction capability, the loss is not offset by gain on the opposite side because the total can only be twice that of the wheel with the lesser capability.
The limited slip differential was designed to improve the traction of a vehicle under adverse traction conditions by allowing the differential to transmit torque to the axle shafts in unequal amounts without interfering with the differential action on turns. The most common limited slip differential is the friction type which has clutch assemblies mounted between the two side gears and the differential case. In a conventional differential, the side gears and the axle shafts to which they are splined always turn freely in the case. The added clutches provide a means of transferring torque from the faster spinning (usually slipping) wheel to the slower spinning (usually better traction) wheel.
Typically, there are one or two clutch packs comprised of disks that are splined to the side gear, and plates that are tanged to fit into the differential case. Thus, the disks rotate with the side gear and the plates rotate with the case. These clutches are often designed to be applied or actuated by hydraulic pressure. This pressure may be regulated as necessary to adjust the differential from zero to full locking as driving needs dictate.
Recent advances in vehicle control may require the disabling of the limited slip feature of the differential at moderate to high speeds. One such system is the yaw stability control which uses the vehicle's brakes to correct the trajectory of the vehicle during a turn. The impulse braking of the yaw stability control feature generates a speed difference between the wheels on either side of the vehicle. The limited slip feature will engage due to this speed difference and may interfere with the performance of the yaw stability control feature. There is therefore a need to disable the limited slip feature of the hydraulic limited slip differential during specified conditions to ensure proper performance of the devices like yaw stability control while also allowing the limited slip feature to be enabled at other specified conditions where traction may be needed and where yaw control is not essential. There is a problem with current hydraulically actuated limited slip differentials in that they do not have a simple on/off capability which is separate and distinct from the hydraulic pressure supply/control circuit actuating the clutch assemblies.
In addition, most hydraulically actuated limited slip differentials employ a passive hydraulic pressure control often in the form of a blow-by seal which releases hydraulic fluid in order to stabilize the pressure at a fixed maximum pressure limit. There currently is no active hydraulic pressure control system which is separate from the hydraulic pressure supply/control circuit which has the ability to regulate the hydraulic pressure between a minimum and a maximum limiting pressure.
It is the intent of this invention to overcome these shortcomings by providing an external control of the hydraulic pressure generated within a hydraulically actuated limited slip differential in which the limited slip clutch can either be turned on or off, or set at any intermediate condition by controlling the maximum system hydraulic pressure limit.